Field of the Invention
The present invention relates to an electron microscope and method of aberration measurement.
Description of Related Art
A scanning transmission electron microscope (STEM) is an instrument for scanning a focused electron beam over a sample, generating a detection signal from electrons transmitted through the sample or from scattering electrons, and mapping the intensity of the detection signal in synchronism with the scanning of the electron beam, thus obtaining a STEM image. In recent years, scanning transmission electron microscopes have attracted attention as electron microscopes capable of providing quite high spatial resolutions at the atomic level. Since the spatial resolution of a scanning transmission electron microscope depends on the diameter of an electron beam impinging on a sample, it is important to reduce the aberration in enhancing the resolution.
In order to obtain high resolution in a short time during observation, it is necessary to previously grasp aberrations induced at that time. For example, JP-A-2012-22971 discloses a method of aberration measurement for use in a scanning transmission electron microscope equipped with a segmented detector. In this known method of aberration measurement, bright-field and dark-field images are derived simultaneously from plural detector segments of the detector located at different positions. Then, aberration coefficients are computed using these bright-field and dark-field images obtained simultaneously. In this method of JP-A-2012-22971, a dark-field image suffering from a less deviation is used as a positional reference and, therefore, the accuracy at which the aberration coefficients are computed can be improved.
In the method of aberration measurement disclosed in the afore-cited JP-A-2012-22971, a segmented detector must be used as noted above. One example of the method capable of measuring aberrations without using a special detector such as a segmented detector is given below.
FIGS. 25 and 26 illustrate one example of the method of aberration measurement for use in a scanning transmission electron microscope. As shown in FIG. 25, in a scanning transmission electron microscope, an electron beam EB is focused onto a sample S by an illumination lens system (not shown). The electron beam EB transmitted through the sample S is detected by a detector 2. A deflector 4 is incorporated in an imaging system. In this scanning transmission electron microscope, as shown in FIG. 26, the detection angle can be controlled by deflecting the electron beam EB by means of the deflector 4.
FIG. 27 is a schematic representation showing a bright-field STEM image I1 obtained under conditions where the electron beam EB is not deflected by the deflector 4. On the other hand, FIG. 28 is a schematic representation showing a bright-field STEM image I2 obtained under conditions where the electron beam EB is deflected by the deflector 4.
As shown in FIG. 25, if a geometric aberration (defocus in the case of FIG. 25) is present in the illumination lens system, the position of impingement of the electron beam EB on the sample S is different for each different angle of convergence and thus the beam does not converge into one point. If the electron beam EB is deflected by the deflector 4, the detector 2 detects rays of the electron beam EB having an angle of incidence (relative to the sample S) corresponding to the amount of deflection. If the angle of incidence relative to the sample S varies, the position of impingement of the beam EB on the sample S deviates according to the amount of aberration in the illumination lens system. Therefore, there occurs shifting of the bright-field STEM image I1 and bright-field STEM image I2 which are formed of the rays of the electron beam EB having mutually different angles of incidence to the sample S as shown in FIGS. 27 and 28. The amount of positional deviation between these bright-field STEM images I1 and I2 corresponds to the aberration in the illumination lens system.
The aberration in the illumination lens system can be calculated from the amount of positional deviation between plural bright-field STEM images obtained by repeating the acquisition of a bright-field STEM image while varying the amount of deflection of the electron beam EB by the deflector 4.
The above-described method of aberration measurement needs acquisition of a number of bright-field STEM images. Furthermore, image drifts occurring during acquisition of the bright-field STEM images are added to the amount of positional deviation between the bright-field STEM images as well as geometric aberration. It takes a long time to acquire such a number of bright-field STEM images and so the above-described method of aberration measurement is greatly affected by image drifts. This makes it difficult to measure aberrations with high accuracy.